This invention relates to screening machines of the type used to separate or classify mixtures of solid particles of different particle sizes into classes of different sizes. The invention also relates to screening machines of the type used for liquid/solid separations, i.e., for separating solid particles of specific sizes from a liquid in which they are carried. More particularly, the invention relates to a structure for mounting components to a vibratory frame of such machines.
In screening machines of the type described, a screen (which may be woven, an aperture plate or another design) is mounted in what is often called a "screen frame" or "screen deck" which includes a supporting peripheral frame or edge around the perimeter of the screen. Typically associated with this screen frame are other material handling elements which are moved with the screen frame and form walls or partitions above or below the screen for containing the liquid and/or particulate materials adjacent to the screen and directing them to appropriate outlets. These elements may comprise a top cover and a pan beneath the screen frame. In the case of multiple deck units, spacer pans or frames are provided between multiple screens. A seal is often provided between the adjacent screen frames to prevent the escape of material from between the screen frames.
The screen frames are often removed from the screening machines for cleaning, replacement, readjustment or installation of a screen of a different mesh size or the like. The top cover and screen frame are each releasably mounted or secured to a frame, table or box to which vibratory motion is imparted, typically by one or more eccentric rotors or other means of vibratory excitation. The frame, table or box is referred to herein as a "vibratory frame". The frame, table or box may be moved in oscillatory, vibratory, gyratory, gyratory reciprocating, fully gyratory, rotary or another type of motion (herein collectively referred to as "vibratory" motion or variations of that term). The vibratory motion of the vibratory frame is typically two-dimensional (i.e., the motion is contained within a plane). The screen assembly is releasably secured to the vibratory frame so that it can be removed for cleaning, change or replacement and also for ease of access to the vibratory drive. Likewise, the top cover is releasably secured by clamps to the vibratory frame to cover the screen frame.
In enlarged commercial screening machines, the weight of the top cover itself or of the screen assembly carried by the vibratory frame, and the weight of the material being processed on it, may total several hundred pounds or more. This presents a very substantial inertial mass which resists the changes of motion applied thereto by the vibratory drive acting through the vibratory frame. As a result of these inertial forces, a relative motion may exist between the vibratory frame and the screening frame or the top cover. Typically, the top cover, screen frame and vibratory frame are each constructed of metal which could result in significant noise, wear or damage due to the relative motion or rubbing action. Reducing the metal-to-metal contact minimizes the wear on the various metal components and the noise associated with the operation of the screening machine. The resulting impact forces between the screen frame or top cover and vibratory frame, which are typically limited to or contained in the two-dimensional plane of the movement of the vibratory frame, would significantly increase the stresses on the components and reduce their useful life.
Screen frame movement is typical because the manufacturing tolerances for many screen frames allow for a 3/8" longitudinal and a 1/2" lateral size difference with respect to the vibratory box frame. Movement of the screen frame and top cover must be minimized to reduce possible wear to these components from the motion of the screen frames. The screen frame and top cover must be secured to the vibratory frame sufficiently such that they essentially follow the vibratory motion of the vibratory frame with rubbing or knocking.
Various devices are known for securing the screen frames to the vibratory frames. One known device for this purpose is disclosed in U.S. Pat. No. 2,114,406 in which a screen deck is clamped against a frame in the form of a box. Movement of the screen frame relative to the vibratory frame is positively prevented by jack screws, commonly called "sieve jacks", which are mounted in the vibratory frame and are set up to bear against a vertical sidewall of the screen frame so that the frame is held in a rigidly fixed position in the vibratory frame. The sieve jacks have proven helpful to reduce the wear of the seals between the adjacent screen frames and to minimize metal-to-metal contact between the screen frames and the vibratory box frame. Sieve jacks are basically threaded rods that penetrate through the screening machine and push the screen frames toward the opposite end of the screening machine to take up the gap from the manufacturing tolerances associated with the screen frames and the vibratory frames.
However, the use of sieve jacks as described has proven to be problematic in many respects. For example, the material being classified or screened in the screening machine often contaminates or fowls the threads of the sieve jacks thereby causing the sieve jacks to bind and resist adjustment or rotation. Additionally, sieve jacks are often used incorrectly so that they are insufficiently tightened to resist screen frame motion, noise and seal wear. Alternatively, many sieve jacks are operated after being overly tightened by the user which may result in screen frame damage, poor sealing between the adjacent screen frames and additional stress to the components. Some of these problems are partly due to the fact that a user cannot visually inspect whether the sieve jacks are engaged with the screen frame after adjustment. Further, a separate tool such as a wrench or the like is required to properly secure the sieve jacks so they do not work themselves loose during operation of the screening machine.
Moreover, the sieve jacks often do not adequately position the screen frames relative to one another due partly to the fact that the user cannot visually inspect the engagement of the sieve jacks. As a result, stack up or alignment problems of the various screen decks is often undetected prior to operation of the screening machine. Therefore, the seals on the adjacent screen frames are misaligned allowing for leakage of the material. Additionally, the sieve jacks are often damaged and require replacement and are considered to be a relatively expensive item.
The top cover has also been known to shift during operation of particularly large commercial screening machines despite the use of multiple, oftentimes 16-20 in number, clamps. As a result, the top cover has heretofore been bolted directly to the box frame to resist movement in the horizontal plane. Even with the use of additional bolts, the clamps are required to maintain downward pressure on the cover which is transmitted downwardly to ensure sealing contact between the adjacent screen frames.
However, the use of bolts is very time consuming for the operators when securing or removing the top cover. Additionally, the integrity of the cover is compromised when the cover is modified to accommodate the bolt which may lead to the escape of particulate material from the screening machine or contamination thereof.